[PowerPC] Fix a warning

This patch fixes:

  llvm/lib/Target/PowerPC/PPCVSXFMAMutate.cpp:293:23: error: unused
  variable 'NewFMAInt' [-Werror,-Wunused-variable]

Author: kazutakahirata

llvm/lib/Target/PowerPC/PPCVSXFMAMutate.cpp

@@ -294,6 +294,7 @@ namespace {
             LIS->createAndComputeVirtRegInterval(KilledProdReg);
 
         LLVM_DEBUG(dbgs() << "  extended: " << NewFMAInt << '\n');
+        (void)NewFMAInt;
 
         // Extend the live interval of the addend source (it might end at the
         // copy to be removed, or somewhere in between there and here). This